Self-healing of thermal cracks in sandwich panels

Sandwich panels are prefabricated, insulated concrete wall elements, which are sensitive to thermal cracking due to their composition (concrete outer cladding – insulation – concrete inner cladding). During hot days, the temperature of the outer concrete layer can rise up to ~ 60°C and the temperature difference between inner and outer layer causes crack formation in the outer concrete layer. Since cracking impairs the durability of concrete (e.g. accelerated corrosion of reinforcement steel by carbonation or chloride ingress), the aim of this research project is to regain impermeability and prevent esthetical damage through incorporation of self-healing capabilities. At first, different healing agents (polyurethane (PU) and water repellent agent (WRA)) were screened based on their ability to regain impermeability and their behaviour upon reloading of cracked and healed samples. Two types of PU and three types of WRA were then selected to be applied in a real scale test. For the real-scale test, the different healing agents were encapsulated by glass capsules and embedded in different zones in the outer layer of a sandwich panel (7.59 m x 1.20 m). After about 14 days, the test setup was built and the outer layer of the self-healing sandwich panel was thermally loaded up to temperatures of ~ 60°C for 9 hours per day. The temperature at the inner layer was kept constant at ~ 21°C. Due to the temperature difference, the panel bended, cracking occurred in the outer cladding, capsules broke and the healing agent was released. Some healing agents leaked out of the crack and left stains behind. Adaptation of the capsule volume, viscosity of the healing agent or concrete cover thickness over the capsules could solve this problem. PU and WRA were able to reduce the water permeability of cracks. Cracks treated with WRA remained water tight upon reloading, while PU can lose their bond with the crack surface resulting in an increased water absorption. In future research, more elastic polyurethanes, with a high bond strength to the concrete matrix, will be tested in order to solve this problem.